Two questions are posed: the origin of antibody variable region diversity and the structure of the thymocyte receptor. The answer to the first question will be sought by isolation of antibody-producing clones from the spleens of inbred mice, both early and late in ontogeny. These clones will be amplified by a cell fusion technique and the amino acid sequence of clonal products determined, utilizing intrinsic radio-labeling and mcirosequencing methods. A comparison of sequences of early and late clones will be employed to determine the specific nature of sequence variation that occurs during maturation. The observation that idiotypic cross-reactivity can be measured between bone marrow and thymus lymphocyte products will be utilized to isolate thymocyte receptors arising in the course of an oligoclonal antibody response in inbred mice. The technique of intrinsic radio-labeling with amino acids of high specific radioactivity will be the basis for determining the complete amino acid sequence of the isolated thymocyte receptor.